Key Takeaways:
- EPA 533 and 537.1 are drinking water-only methods. EPA 1633 is the only option for wastewater, groundwater, soil, biosolids, and other non-drinking-water matrices.
- Method 533 catches short-chain PFAS and precursor compounds that 537.1’s chemistry handles less reliably. Method 537.1 carries the longer monitoring history for legacy long-chain compounds.
- Federal rule 40 CFR 141.901(b)(1) accepts either 533 or 537.1 for drinking water compliance. Certification and analyte priorities decide which one a lab runs.
- Method 1633 is the method EPA points to for Clean Water Act discharge monitoring and non-drinking-water site investigation work.
- The method has to be right before the sample is taken. Once it’s prepped and analyzed, it can’t be swapped after the fact.
A client asks for PFAS testing and expects one method, one number, one answer. In practice, a report can come back referencing EPA Method 533, EPA Method 537.1, or EPA Method 1633, and the difference between them isn’t a formality. It determines which compounds get measured, whether the result is valid for the sample’s matrix, and whether it can be used for regulatory compliance at all.
Three methods, Three Different Regulatory Jobs.
The starting point for understanding these methods is the program each one serves, not the chemistry.
Methods 533 and 537.1 exist for one purpose: analyzing finished drinking water under the Safe Drinking Water Act. Both are approved methods for the federal PFAS National Primary Drinking Water Regulation, and both were used to collect national occurrence data under EPA’s Fifth Unregulated Contaminant Monitoring Rule (UCMR5).
Method 1633 serves a different program entirely. It’s validated for wastewater, surface water, groundwater, soil, sediment, biosolids, landfill leachate, and fish tissue, and it’s the method EPA points to for Clean Water Act discharge monitoring and non-drinking-water site investigation work.
A drinking water method and a wastewater method aren’t interchangeable just because both measure PFAS. The extraction chemistry in 533 and 537.1 can struggle with the suspended solids typical of wastewater, and neither method includes the cleanup steps Method 1633 requires for those matrices. A lab offering “modified 537.1” or “modified 533” for wastewater samples is running an in-house procedure loosely based on the original method, not a validated one. Neither drinking water method permits modification in the first place.
What Separates 533 from 537.1
For drinking water specifically, the choice between 533 and 537.1 makes a difference.
Method 533 uses weak anion exchange solid-phase extraction and targets 25 PFAS. It’s built to catch compounds 537.1 handles less reliably: short-chain PFAS, fluorotelomer sulfonates, and precursor compounds like HFPO-DA and ADONA that don’t extract well with 537.1’s sorbent chemistry.Â
Method 537.1 uses styrene-divinylbenzene extraction and targets 18 PFAS. It carries the longer monitoring history as a direct descendant of the original Method 537, and it remains the stronger choice for long-chain legacy compounds where continuity with historical data matters.
Together, the two methods cover 29 unique PFAS. Neither one alone catches everything the other does.
Why a Lab Picks One Method Over the Other
For drinking water compliance, federal rule 40 CFR 141.901(b)(1) accepts either Method 533 or Method 537.1. The choice between them comes down to a few practical factors:
- Analyte coverage. A client whose priority compounds skew short-chain gets better coverage from 533. A client focused on legacy long-chain PFAS may be well served by 537.1 alone.
- Certification and instrumentation. Lab certification is method-specific. Which method a lab is accredited and instrumented for often decides what gets offered by default.
- Regulatory driver. If the sample isn’t drinking water at all, none of this applies. Method 1633 is the only option, and results from 533 or 537.1, modified or not, won’t hold up for Clean Water Act compliance reporting.
The Three Methods At-a-Glance
| EPA 533 | EPA 537.1 | EPA 1633(A) | |
| Matrix | Finished drinking water only | Finished drinking water only | Wastewater, surface water, groundwater, soil, sediment, biosolids, landfill leachate, fish tissue |
| Analyte Count | 25 PFAS | 18 PFAS | 40 PFAS |
| Extraction Chemistry | Weak anion exchange (WAX) SPE | Styrene-divinylbenzene (SDVB/C18) SPE | WAX SPE, plus a solvent extraction step first for solids |
| Governing Program | SDWA — UCMR5, NPDWR compliance | SDWA — UCMR5, NPDWR compliance | CWA — NPDES permits, biosolids, site investigation |
| Published | 2019 | Revision of 537 (v2.0, March 2020) | Finalized as 1633A, December 2024 |
Get the Method Right Before You Sample
The method printed on a PFAS report tells you what was measured, whether the result is valid for the sample matrix, and whether it can be used where regulatory compliance is on the line. Once a sample is prepped and analyzed, the method can’t be changed, so knowing what’s needed before the sampling kit goes out is what keeps results defensible data instead of a rerun.
Your lab project manager and technical team can walk through which method applies to your matrix and your program before you order.
Frequently Asked Questions
Can I use EPA Method 533 or 537.1 for a wastewater sample?
No. Both are validated for finished drinking water only. Wastewater, groundwater, and soil samples require EPA Method 1633, which includes cleanup steps 533 and 537.1 don’t have for those matrices.
Is EPA Method 533 better than 537.1?
Neither is universally better. Method 533 covers more short-chain PFAS and certain precursor compounds. Method 537.1 has a longer monitoring history for long-chain legacy PFAS. Together they cover 29 unique compounds, and which one applies depends on your compliance program and priority analytes.
Why does my drinking water report say EPA 537.1 instead of 533?
Labs are certified per method, so the one run is often the one a facility is accredited and instrumented for. Both 533 and 537.1 satisfy 40 CFR 141.901(b)(1) for drinking water compliance.
What is EPA Method 1633 used for?
Method 1633 is EPA’s method for non-drinking-water PFAS testing, covering wastewater, surface water, groundwater, soil, sediment, biosolids, landfill leachate, and fish tissue. It’s the method referenced for Clean Water Act discharge monitoring and site investigation work.
Can a lab modify EPA 533 or 537.1 to test wastewater?
No. A “modified 533” or “modified 537.1” offering is an in-house procedure, not a validated EPA method, and results from it won’t hold up for Clean Water Act compliance reporting.
How many PFAS compounds does each method detect?
EPA 533 targets 25 PFAS, EPA 537.1 targets 18 PFAS, and EPA 1633(A) targets 40 PFAS across a broader range of matrices.
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